Research On Sedimentary Facies Of Shanxi Formation Of Permian System In Zizhou Gas Field

Ordos Basin is the second largest basin with oil and gas. It has the character of distribution pattern, Mesozoic having oil, Paleozoic having gas, superficial part having oil, deep part having gas. Zizhou Gas Field is in the east of Shanbei Slope in Ordos Basin, and the primary gas-bearing bed is Shanxi Formation of Permian System. In order to serve exploration and development more preferably, by means of sedimentology, reservoir sedimentology, petroleum geology, analyzing and testing measures, the more systemic researches have been carried out for sedimentary facies of Shanxi Formation.On the basis of stratigraphic classification of the upper Paleozoic in Ordos Basin, combined with key bed, lithologic character and electric property character, cycle of sedimentation, etc, Shanxi Formation are divided into six gas-bearing formations: Shan 23, Shan 22, Shan 21, Shan 13, Shan 12, Shan 11. By the studying of ancient land distribution feature, the light and heavy minerals, the paleocurrent analysis as well as geochemistry analysis, it makes sure that the north is the main direction of sediment provenance, the original rock is metamorphic rock of Eozoic in the north of Ordos Basin.According to sedimentological marker, alaeontologic marker, geophysical marker, and geochemical marker, it is found that delta front existed in Zizhou Gas Field. Delta front can be devided into four microfacies, distributary channel underwater, interdistributary, riedel swamp and sand sheet. Sedimentary facies controled the spreading of sand bodies, and they spreaded from north to south integrally. Compared with every gas-bearing formation, the character of lake expanding and river atrophying subsisted from Shan 23 to Shan 11. Shan 23 gas-bearing formation has the largest sand bodies, and it is the main gas-producing zone in Zizhou Gas Field.Palaeogeomorphology of the bottom of Shanxi Formation is researched, using moulage method of board thickness after V-cut and thickness compensation, the method of peeling back and filling up. Based on grain volume and mass balance law, the paleogeomorphogloic map of the bottom of Shanxi Formation is recovered after decompaction correction of bed thickness of Shan 2 member. The paleogeomorphogloic map shows that hypsography of the bottom of Shanxi Formation is subdued comparatively. Channel underwater exists in the low-lying parts. Interdistributary lies in the flatness. River levee is being in the higher land.Slope belt is the important area to look for subtle reservoir, and so it is necessary to research slope belt in sedimentary basin. Confering predecessors' ideas and method, slope belt is studied based on palaeogeomorphology research. Gradual slope belt subsists in Zizhou Gas Field. In course of forming, it was controlled by malconformation of sedimentation and compaction.